In machine tools for machining a workpiece with a rotating tool, chattering vibrations may be generated by machining, i.e., a process for chipping a workpiece by cutting edges of a rotating tool. Generally, the chattering vibrations are more likely generated when a hard-to-cut material is machined. Chattering vibrations deteriorate the machining accuracy of a workpiece. Chattering vibrations often result from insufficient rigidity of a transmission system due to backlash of gear wheels of a reduction drive used in a feed axis device of a machine tool.
In order to reduce chattering vibrations, sometimes the cutting condition, such as the rotational speed of a tool or the feeding speed, must be changed. This method usually involves reduction in machining efficiency. Therefore, Patent Literature 1 discloses a rotary feed axis device having two parallel gear trains for driving a single feed axis with driving forces from two servomotors. In the invention of Patent Literature 1, the two servomotors are controlled so as to increase the pressers between surfaces of engaging teeth of the two gear trains of the rotary feed axis device, when a sensor means, for detecting the vibrations generated in a machine tool machining a workpiece, detects vibrations.
Further, as a technology for removing backlash, Patent Literature 2 describes a driving apparatus for an industrial robot having a robot arm supported by two reduction drives. In the invention of Patent Literature 2, in order to remove backlash, the input shaft of one of the two reduction drives is connected to the input shaft of the other reduction drives.